10. FUNDAMENTAL LESSONS

More important than the precise values of parameters is what we have
learned about the general features which describe our observable Universe.
Beyond the basic hot Big Bang picture, the CMB has taught us that:

The Universe recombined at
z 1100 and
started to become ionized again at
z 10-30.

The geometry of the Universe is close to flat.

Both Dark Matter and Dark Energy are required.

Gravitational instability is sufficient to grow all of the observed
large structures in the Universe.

Topological defects were not important for structure formation.

There are `synchronized' super-Hubble modes generated in the early
Universe.

The initial perturbations were adiabatic in nature.

The perturbations had close to Gaussian (i.e., maximally
random) initial conditions.

It is very tempting to make an analogy between the status of the
cosmological `Standard Model' and that of particle physics. In cosmology
there are about 10 free parameters, each of which is becoming well
determined, and with a great deal of consistency between different
measurements. However, none of these
parameters can be calculated from a fundamental theory, and so hints of the
bigger picture, `physics beyond the Standard Model' are being searched for
with ever more challenging experiments.

Despite this analogy,
there are some basic differences. For one thing, many of the cosmological
parameters change with cosmic epoch, and so the measured
values are simply the ones
determined today, and hence they are not `constants', like particle
masses for example (although they are deterministic, so that if one
knows their values at one epoch, they can be calculated at another).
Moreover, the number of parameters is not as fixed as it is in
the particle physics Standard Model; different researchers will not
necessarily agree on what the free parameters are, and new ones
can be added as the quality of the data improves. In addition
parameters like , which come
from astrophysics,
are in principle calculable from known physical processes, although this
is currently impractical. On top of all this, other parameters might be
`stochastic' in that they may be fixed only in our observable patch
of the Universe.

In a more general sense the cosmological `Standard
Model' is much further from the underlying `fundamental theory' which
will provide the
values of the parameters from first principles. On the other hand, any
genuinely complete `theory of everything' must include an explanation for
the values of these cosmological parameters as well as the parameters of
the Standard Model.